The present invention concerns a protective structure for vehicles designed to be used, in particular, in the event of impact with pedestrians.
The recent legislative proposals concerning impact in the automotive field provide for increasingly stringent limits in particular as regards impact occurring between vehicles and pedestrians (so-called pedestrian impact).
In this regard numerous absorption systems have been studied for the purpose of observing said limits without affecting performance on other types of impact.
In particular, the conditions proposed by the current regulations provide for maximum leg rotation in the event of impact to be less than or equal to 15xc2x0 at average-low vehicle speed (approximately 40 km/h), while maximum displacement between femur and tibia must be less than or equal to 6 mm; a further maximum limit is established (less than or equal to 150 g where g represents the acceleration of gravity, equal to approximately 9.81 m/s2) in relation to acceleration values corresponding to the tibia.
The test measurements performed, on the basis of the data relating to the mean weight of a femur and a tibia and at the speed of a vehicle during impact, result in a final energy balance in which, according to the style of the vehicle bodywork, the end of the impact is ideally determined when the initial energy of the leg is converted into work due to the movements of the same (shearing force work, work due to the rotation moment and deformation work due to crushing of the absorption devices).
From this, and from the limits on the rotation and shearing movements established by the regulations, it derives that 90% of the energy is absorbed by the deformation work and, for this reason, it is necessary to use inside the bumper strip of the vehicle at least one energy absorption device or buffer sized so that the crushed portion is equal to at least 70 mm (taking account of the average shape of the vehicles currently produced).
It should be noted that in the case of a flat vehicle front (although so far a shape of this type has never been designed) the length of the buffer would have to be such as to ensure a crushed portion equal to at least 35 mm; this clearly leads to the consideration that one of the fundamental variables in the design of absorption systems complying with the limitations established by the regulations, in the event of pedestrian impact, is the shape of the vehicle which, however, is sometimes difficult to modify due to aesthetic and marketing factors.
Furthermore, using absorption systems of known types, finite element analysis shows that with buffers of different densities, the maximum impact acceleration and the maximum rotation angle of the leg increase with the compactness and hardness of the absorption device; also in this case, therefore, the limits established by the regulations are substantially exceeded.
Another important factor to be taken into consideration is that ideal absorption devices do not exist in nature, i.e. such that a given force is constant throughout the entire buffer crushing range.
On the contrary, the existing absorbers (normally made of foamed polypropylene or polyurethane foam or extruded polypropylene foam, such as strand foam(copyright)) do not perform constantly over the entire crushing range.
The test measurements relating to a pedestrian impact simulation performed with traditional type absorber devices (consisting of a buffer fitted between bumper strip and forced to work on a resistant cross member) have shown that said devices cannot satisfy the requirements unless the structure has considerable overall dimensions, with respect to a possible minimum theoretical value of approximately 70 mm.
Further tests were performed both in normal conditions with the bumper brought forward in the direction of the impact point by approximately 100 mm; also in these cases, however, the maximum values relating to impact acceleration, rotation corresponding to the knee and maximum shearing, although slightly different, were well above the required limits.
It is therefore evident that the increase in the market demand for attractively designed good-looking motor vehicles conflicts with considerations relating to the safety of pedestrians who, in the event of impact with vehicles travelling at average-low speeds, must be safeguarded, with a reasonable hope of avoiding serious or even permanent injury.
On the other hand, if the absorber devices were arranged longitudinally between bumper and cross member, the thickness of the buffer would have to be increased out of all proportion with respect to the rules of style for vehicle profiles and at the expense of the weight, overall costs of the structure and overall performance of the vehicle, in order to satisfy safety requirements, both in the case of impact with bulky heavy obstacles (road signs, other vehicles etc.) and in the case of collision with pedestrians.
The aim of the present invention is therefore to eliminate the above disadvantages, producing a highly efficient protective structure for vehicles, designed to be used particularly in the event of impact with pedestrians which, first and foremost, meets the requirements of the current regulations concerning collision of a vehicle with a pedestrian at average-low speed, without modifying the shape, the style and the performance of the vehicle on other types of impact.
A further aim of the invention is to produce a protective structure for vehicles, designed to be used in particular in the event of impact with pedestrians, which is able to considerably increase the force on the leg of the pedestrian without increasing the rotation produced, at the same time featuring compact dimensions of the energy absorber devices used.
These and other aims, according to the present invention, are achieved by producing a protective structure for vehicles, designed to be used particularly in the event of impact with pedestrians, according to claim 1, here referred to for the sake of brevity.
Advantageously, the structure subject of the present invention has more limited overall dimensions and is considerably cheaper and more effective in terms of performance than the known structures.
In particular, by using the absorption structure according to the present invention, the force on the leg of the pedestrian can be increased without increasing the rotation force on the knee.